Structure of ferrichrome-type siderophores with dissimilar N delta-acyl groups: asperchrome B1, B2, B3, D1, D2 and D3.

Asperchromes are a series of iron-chelating compounds which contain a cyclic hexapeptide backbone as in ferrichrome siderophores and differ from the latter in having heterogenous acyl groups in the ornithine side chains. The molecular structures of the asperchrome B and D series have been determined by 1H- and 13C-NMR spectroscopy; single-crystal X-ray diffraction was used to determine the detailed structural features of asperchrome B1 and asperchrome D1. Asperchrome B1 crystallizes in the triclinic space group P1 with a = 1.3143(5) nm, b = 1.2200(5) nm, c = 0.8949(3) nm, alpha = 105.17(4) degrees, beta = 94.03(3) degrees, gamma = 109.65(3) degrees, V = 1.2843 nm3, Z = 1, rho chi = 1.446 g cm-3. Final R = 0.054 for 4625 reflections measured at 138 K using MoK alpha. Asperchrome D1 crystallizes in the monoclinic space group P2(1) with a = 1.2248(11) nm, b = 1.3795(9) nm, c = 1.3644(6) nm, beta = 93.24(6) degrees, V = 2.3016 nm3, Z = 2, rho chi = 1.418 g cm-3. Final R = 0.110 for 3180 reflections measured at 138 K using MoK alpha radiation. The conformation of the molecular backbone and iron coordination geometry in both asperchrome B1 and D1 compare well with those observed in other known ferrichrome siderophores. The differences in the acyl groups are illustrated and the structural results are correlated with their iron transport properties.